Method for flattening surface of oxide crystal to ultra high degree

ABSTRACT

There are provided a method of superflattening an oxide crystal that is soluble neither with acid nor with alkaline, a method of making a ReCa 4 O(BO 3 ) 3  family oxide single crystal thin film using the superflattening method, a ReCa 4 O(BO 3 ) 3  family oxide single crystal thin film having a SHG property, a superflattening method for light incident/emitting surfaces, and a defect assessing method for oxide crystals. The surface of an oxide crystal that is soluble neither with acid nor with alkaline is reduced with a reducing agent, the reduced oxide crystal surface is dissolved with an aqueous solution of acid or alkaline, the surface dissolved oxide crystal is heat-treated in the atmosphere, whereby the surface of an oxide crystal that is soluble neither with acid nor with alkaline is superflattened to an atomic level. According to this method, a chemically stable oxide which because of its complexity in both composition and structure is soluble neither with acid nor with alkaline and is insoluble even with a fluoric acid is allowed by reduction to be converted into a simpler oxide conventionally soluble with hydrochloric, nitric or sulfuric acid; hence a surface of its crystal is rendered capable of dissolving. Then, heat-treating the dissolved surface in the atmosphere at a suitable temperature for a suitable time period allows surface atoms to be rearranged and the surface to be superflattened to an atomic level. The present invention is applicable to the technical fields that require ultraviolet laser light, especially as core technologies of optical devices applied to optical information processing, optical communication or the like.

TECHNICAL FIELD

[0001] The present invention relates to a superflattening method foroxide crystal surfaces soluble neither with acid nor with alkaline, amethod of making a ReCa₄O(BO₃)₃ system oxide single crystal thin filmusing that method and a ReCa₄O(BO₃)₃ system oxide single crystal thinfilm made thereby.

[0002] The present invention further relates to a superflattening methodfor a light incident/emitting surface of oxide optical crystal and adefect assessing method for oxide crystals in which the abovementionedsuperflattening method for oxide crystal surfaces is applied.

BACKGROUND ART

[0003] In recent years demands for shortwave lasers are increasing inthe fields of laser machining, laser medical treatment, material surfacereforming and optical information processing, and their vigorousresearch and development are in progress. Since semiconductor lasers areonly capable of generating light from the infrared to blue regions,however, it is essential to adopt a wavelength conversion technique forlight ultra-violetization. As such wavelength conversion techniques, SHG(second harmonic generation) and THG (third harmonic generation)utilizing the nonlinear optical effect of an optical crystal are in use.While such crystals as composed of KTP (KTiOPO₄) and KDP (KH₂PO₄) haveso far been used as crystal bringing about the nonlinear optical effect,these crystals have problems in that their refractive index fluctuates,their absorption loss is large, they are liable to suffer from opticaldamages, they are water soluble and hence poor in resistance toenvironment, their nonlinear optical effect is not much satisfactoryand/or their thermal conductivity is poor.

[0004] Amongst the attempts to find a nonlinear optical effect crystalthat overcomes these shortcomings of KTP and KDP crystals, there hasrecently been synthesized, as disclosed in JP H10-206916 A, a bulksingle crystal called rare-earth/calcium/oxyborate [ReCa₄O(BO₃)₃ whereRe is one or more rare-earth elements] for use in SHG (second harmonicgeneration) and THG (third harmonic generation) of Nd: YAG laser light.Being an oxide optical crystal, this crystal found to get over thedifficulties of KTP and KDP crystals and to exhibit extremely high SHGand THG efficiency is considered an optical crystal that shouldconstitute the nucleus of the coming wavelength conversion technique.

[0005] However, despite high demand for its application especially inthe field of optical information processing, it has so far beenunsuccessful to make this crystal in the form of a thin film because ofits crystallographic structure which is extremely complicated.

[0006]FIG. 6 shows the crystallographic structure of ReCa₄O(BO₃)₃ whereRe is one or more rare-earth elements, wherein FIG. 6A is a typicalatomic view of this crystal seen from the direction of its b axis, andFIG. 6B is a typical atomic view of the same seen from the direction ofits c axis. FIG. 6C shows its basic unit lattice, indicating that thiscrystal is a monoclinic biaxial crystal belonging to point group m andspace group Cm and has its lattice constants a: about 8.09 angstroms, b:about 16. 01 angstroms, and c: about 3.56 angstroms, although precisevalues vary depending on the type and amount of Re.

[0007] As shown, this crystal is extremely large in the number of theatoms making up the basic unit lattice, extremely large in latticeconstant and is complicated in structure.

[0008]FIG. 7 shows results of the X-ray diffraction measurement of athin film formed by trying to epitaxially grow ReCa₄O(BO₃)₃ where Re=Gd,Y upon a STO (strontium titanate) and an Al₂O₃ (alumina) single crystalsubstrate by laser ablation. As is evident from the diffraction patternsshown, a ReCa₄O(BO₃)₃ thin film epitaxially grown is not obtained.

[0009] Since ReCa₄O(BO₃)₃ is extremely complex in the crystallographicstructure, epitaxially growing a single crystal thin film thereof upon asubstrate requires that the substrate be similar thereto incrystallographic structure and have its surface flattened to an atomiclevel. Hard to fulfill the requirement of so flattening a surface, theprior art has not be successful in giving rise to such a single crystalthin film.

[0010] Thus, the extremely complex crystallographic structure of crystalReCa₄O(BO₃)₃ has not permitted the state of the art to yield a singlecrystal thin film thereof and hence to make its nonlinear opticalproperty to be exploited in a thin film structure.

[0011] Also, the SHG and THG cannot be carried out efficiently unlessthe light incident/emitting surface of an optical crystal is flattenedto the extent of a wavelength or less. The use in the prior art of anabrasive of a wavelength or less in size to polish such a surface hasmade it unavoidable that an irregularity in the order of its grain sizeor a small crack is formed in the surface. As a result, such a surfaceserving especially to emit high-energy photons may have various opticaldamages arising from a partial breakage thereof. Preventing suchphenomena from taking place requires the structural integrity of thelight incident/emitting surface of an optical crystal, which hashitherto been achieved by polishing only to a limited degree, however.

[0012] Also, where the quality of an optical crystal is to be assessed,a method may be adopted in which defect portions of the optical crystalare selectively etched and the number of resulting etch pits are countedto assess its quality. If the optical crystal is such an oxide crystalas ReCa₄O(BO₃)₃ which is soluble neither with acid nor with alkaline,the problem is imposed that its quality assessment can by no means beattained conventionally; such an oxide crystal has no means to assessits quality.

[0013] With these problems taken into account, it is a first object ofthe present invention to provide a method of superflattening a surfaceof an oxide crystal soluble neither with acid nor with alkaline.

[0014] Another object of the present invention is to provide a method ofmaking a ReCa₄O(BO₃)₃ family oxide single crystal thin film using such amethod and a ReCa₄O(BO₃)₃ family oxide single crystal thin film madethereby.

[0015] Further objects of the present invention are to provide a methodof superflattening a light incident/emitting surface of an oxide opticalcrystal and a surface defect assessment method for an oxide crystal inboth of which methods the method as the first object is applied.

DISCLOSURE OF THE INVENTION

[0016] Oxides depending on the nature of an aqueous solution with whichit is soluble can be classified into three types, viz. an “acid oxide”which is soluble with an alkaline solution, a “basic oxide” which issoluble with an acidic aqueous solution, and an “amphoteric oxide” whichis soluble with both acid and alkaline. An oxide that is strong inionicity is a basic oxide and generally is soluble with an acid such ashydrochloric, nitric and sulfuric acids. As for oxides which are complexin both composition and structure, there exists an oxide crystal whichis soluble neither with acid and nor alkaline and further not even withfluoric acid, and hence extremely stable chemically.

[0017] Such an oxide can be rendered soluble as follows: To wit, sincean oxide is in the state that a metal is oxidized and hence can bereduced, it can be decomposed by reduction using a reducing agent suchas oxalic acid or an aqueous solution of sodium thiosulfate to form, oris thereby converted into, a simpler oxide in which metal bonds tooxygen of water.

[0018] Since a simpler oxide once converted into can be renderedrelatively soluble with an acidic or alkaline aqueous solution, it ispossible to dissolve a surface of the oxide crystal conventionally withhydrochloric, nitric or sulfuric acid.

[0019] The surface dissolved having a state that dangling bonds areexposed, the heat-treatment in the atmosphere at a suitable temperaturefor a suitable time period allows its surface atoms to be rearranged sothat the surface free energy is minimized; hence the surface issuperflattened to an atomic level.

[0020] There is thus provided in accordance with the present invention amethod of superflattening a surface of an oxide crystal that is solubleneither with an acid nor with alkaline, characterized in that itcomprises the steps of: reducing the surface of the oxide crystal with areducing agent; dissolving the reduced oxide crystal surface with anaqueous acid or alkaline solution; and heat-treating in the atmospherethe oxide crystal with its reduced surface dissolved, whereby a surfaceof the oxide crystal soluble neither with acid nor with alkaline isflattened on an atomic level.

[0021] According to this method, a chemically stable oxide which becauseof its complexity in both composition and structure is soluble neitherwith acid nor with alkaline and is insoluble even with a fluoric acid isallowed by reduction to be converted into a simpler oxide conventionallysoluble with hydrochloric, nitric or sulfuric acid; hence a surface ofits crystal is rendered capable of dissolving. Then, heat-treating thedissolved surface in the atmosphere at a suitable temperature for asuitable time period allows surface atoms to be rearranged and thesurface to be superflattened to an atomic level.

[0022] The present invention also provides a method of making aReCa₄O(BO₃)₃ system oxide single crystal thin film, characterized inthat it comprises the steps of: reducing with a reducing agent a surfaceof an oxide single crystal having a composition expressed by chemicalformula: ReCa₄O(BO₃)₃ where Re represents one or more rare earthelement; dissolving the reduced oxide single crystal surface with anaqueous acid or alkaline solution; heat-treating in the atmosphere theoxide single crystal with its reduced surface dissolved, therebysuperflattening a surface thereof; and then epitaxially growing aReCa₄O(BO₃)₃ thin film on the superflattened surface.

[0023] According to this method, a surface of ReCa₄O(BO₃)₃ family oxidesingle crystal that is extremely complex in both composition andstructure is allowed to be superflattened, and the superflattenedsurface allows epitaxially growing a ReCa₄O(BO₃)₃ family oxide singlecrystal thin film thereon.

[0024] The present invention also provides an oxide single crystal thinfilm having a composition expressed by chemical formula: ReCa₄O(BO₃)₃where R is one or more rare earth elements, characterized in that it isa thin film of said composition epitaxially grown on a surface of anoxide single crystal having a composition expressed by chemical formula:ReCa₄O(BO₃)₃ and superflattened.

[0025] The oxide single crystal thin film mentioned above mayspecifically be an oxide single crystal thin film characterized byhaving a composition expressed by chemical formula: ReCa₄O(BO₃)₃ andwhich is a single crystal thin film that has an monoclinic biaxialcrystalline structure belonging to point group m and space group Cm.

[0026] The oxide single crystal thin film mentioned above mayspecifically be an oxide single crystal thin film characterized byhaving a nonlinear optical property, thus providing SHG and THGproperties.

[0027] The present invention also provides a method of flattening alight incident/emitting surface, characterized in that the surface is ofan oxide optical crystal soluble neither with acid nor with alkaline andthe method comprises the steps of: reducing a light incident/emittingsurface of the oxide optical crystal with a reducing agent; dissolvingthe reduced oxide optical crystal surface with an aqueous acid oralkaline solution; and heat-treating in the atmosphere the oxide opticalcrystal with its surface dissolved, to thereby flatten the lightincoming and outgoing surface, whereby a light-incident/emitting surfaceof the oxide optical crystal is flattened.

[0028] According to this method, a light incident/emitting surface of anoxide optical crystal is allowed to be superflattened and is therebyprevented from having breakage of the surface and optical damages ingenerating harmonics in SHG and THG.

[0029] The present invention further provides a crystal defectassessment method, characterized in that it comprises the steps of:reducing with a reducing agent a surface of an oxide crystal which issoluble neither with acid nor with alkaline; and dissolving defects inthe reduced oxide crystal surface with an etching solution for etchingthe said defects selectively, thereby forming etch pits of the oxidecrystal for assessment of the crystal defects. According to this method,it becomes possible to assess the quality of an oxide crystal solubleneither with acid nor with alkaline which has so far been hard toassess.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The present invention will better be understood from thefollowing detailed description and the drawings attached hereto showingcertain illustrative forms of embodiment of the present invention. Inthis connection, it should be noted that such forms of embodimentillustrated in the accompanying drawings hereof are intended in no wayto limit the present invention but to facilitate an explanation andunderstanding thereof. In the drawings:

[0031]FIG. 1 diagrammatically shows results of measurement of thesurface flatness when a superflattening method for oxide crystalsurfaces according to the present invention is applied to GdCa₄O(BO₃)₃;

[0032]FIG. 2 is a graph showing results of measurement of the surfaceflatness taken under varying heat-treatment conditions in thesuperflattening method for oxide crystal surfaces of the presentinvention;

[0033]FIG. 3 shows graphically results of RHEED measurement taken duringthe growth of a Gd_(1-x)Y_(x)Ca₄O(BO₃)₃ thin film in a method of makinga ReCa₄O(BO₃)₃ family oxide single crystal thin film in accordance withthe present invention;

[0034]FIG. 4 is a graph showing minimum substrate temperatures requiredfor the epitaxial growth of a single crystal film for differentrare-earth elements;

[0035]FIG. 5 shows results of measurement of the SHG property of anoxide single crystal thin film having a composition expressed bychemical formula: ReCa₄O(BO₃)₃ where Re is one or more rare-earthelements, in accordance with the present invention;

[0036]FIG. 6 diagrammatically shows the crystallographic structure of acomposition ReCa₄O(BO₃)₃ where Re is one or more rare-earth elements;and

[0037]FIG. 7 shows results of X-ray diffraction measurement taken whenthin films of compositions: ReCa₄O(BO₃)₃ where Re=Gd and Y are tried togrow epitaxially by molecular beam epitaxy on a STO (strontium titanate)and an Al₂O₃ (alumina) single crystal substrate.

BEST MODES FOR CARRYING OUT THE INVENTION

[0038] Hereinafter, the present invention will be described in detailwith reference to suitable forms of implementation thereof illustratedin the drawing figures.

[0039] Mention is first made of a superflattening method for oxidecrystal surfaces according to the present invention.

[0040] GdCa₄O(BO₃)₃ (gadolinium/calcium/oxyborate) crystal is an oxidecrystal that is soluble neither with acid nor with alkali and itscrystallographic structure is identical to that shown in FIG. 5.

[0041] A bulk single crystal of GdCa₄O(BO₃)₃ was superflattened in thesteps stated below.

[0042] 1) Dipping the bulk single crystal in an aqueous solution ofoxalic acid (0.5 mol/l) as a reducing agent for a time period of 10 to30 seconds to convert a surface thereof into a simpler oxide;

[0043] 2) Subjecting the reduced bulk single crystal surface toultrasonic cleaning in pure water for a time period of 3 minutes;

[0044] 3) Dipping the cleaned bulk single crystal in an aqueous solutionof hydrochloric acid (5×10 4 mol/cm³) as an acid dissolving thereduction formed simpler oxide for a time period of 10 to 30 seconds;

[0045] 4) Subjecting the resultant bulk single crystal to ultrasoniccleaning in pure water for a period of 3 minutes; and

[0046] 5) With an electric furnace, heat-treating the resultant bulksingle crystal in the atmosphere at a temperature of 1000° C. for a timeperiod of 10 hours, thereby obtaining a superflattened bulk singlecrystal.

[0047] The GdCa₄O(BO₃)₃ crystal surface thus superflattened was measuredwith an atomic force microscope (AFM) as to its flatness.

[0048]FIG. 1 diagrammatically shows results of measurement of thesurface flatness when a superflattening method for oxide crystalsurfaces according to the present invention is applied to a GdCa₄O(BO₃)₃bulk crystal.

[0049]FIG. 1(A) shows an AFM image of a (010) crystallographic surfaceof the GdCa₄O(BO₃)₃ bulk crystal taken before the superflattening methodfor oxide crystal surfaces according to the present invention isapplied, namely after the substrate is polished, and FIG. 1(B) shows aresult of measurement of surface roughness along the horizontal line inFIG. 1(A).

[0050] From these Figures, it is seen that the surface has polishingimpressions and is not flattened to an atomic level.

[0051]FIG. 3(C) shows an AFM image of the f a (010) crystallographicsurface of the GdCa₄O(BO₃)₃ bulk crystal taken after the abovementionedsteps 1) to 5) are carried out, and FIG. 1(D) shows results ofmeasurement of surface roughness along the horizontal line in FIG. 1(A).

[0052] From these Figures, it is seen that the GdCa₄O(BO₃)₃ bulk crystalsurface has its (010) crystallographic face formed as terraced along itscrystallographic b-axis with an identical step of about 8 angstroms(which corresponds to about a half of the lattice constant in thedirection of the b-axis).

[0053]FIG. 2 shows results of a test in which the relationship betweenthe heat-treatment conditions in step 5) and the resultant surfaceunevenness. In the graph of FIG. 2, the left hand side ordinate axisrepresents the surface roughness, the right hand side ordinate axisrepresents the maximum unevenness, and the abscissa axis represents theheat-treatment temperature. From this graph, it is seen thatestablishing suitable heat-treatment conditions allows the surface to besuperflattened to 5 angstroms or less, namely to an atomic level.

[0054] Mention is next made of a method of making a ReCa₄O(BO₃)₃ familyoxide single crystal thin film in accordance with the present inventionand a ReCa₄O(BO₃)₃ family oxide single crystal thin film made thereby,reference being made to a specific example as follows:

[0055] 1) A (010) crystallographic surface of a GdCa₄O(BO₃)₃ crystalsubstrate is used which having undergone the superflattening steps foroxide single crystal surfaces according to the present invention issuperflattened.

[0056] 2) For growing a thin film, use is made of laser ablation MBEequipment with a substrate temperature of 500 to 700° C., a laser energyof about 5 joules/cm², a laser repetition rate of 1 to 20 Hz, an oxygenpartial pressure of about 1×10⁻⁶, the target made of aGd_(1-x)Y_(x)Ca₄O(BO₃)₃ single crystal, and a substrate to targetdistance of about 5 cm.

[0057] Mention is made of results of RHEED (reflection high-energyelectron diffraction) measurement during the film growth.

[0058]FIG. 3 shows graphically results of RHEED measurement taken duringthe growth of a Gd_(1-x)YxCa₄O(BO₃)₃ in a method of making aReCa₄O(BO₃)₃ family oxide single crystal thin film in accordance withthe present invention.

[0059]FIG. 3(A) shows RHEED intensity oscillations during the filmgrowth. In the Figure, the abscissa axis represents the lapse of timewith the film growth starting point of time as its origin, and theordinate axis represents the RHEED intensity. Each of the hatched areasindicates a film growing, the frequency shown in each hatched areaindicates a laser repetition rate then used, and each unhatched or whitezone represents a preparatory period for a next film growth in which thefilm growth conditions are altered and there is no film growth. Thesmall graph shown in FIG. 3(A) shows as enlarged a portion of the RHEEDoscillations which occur with the laser beam turned on and off. As isapparent from changes in shape of envelop curves indicating RHEEDintensity oscillatory waveforms, it is seen that the thin filmepitaxially grows for each one molecular layer.

[0060]FIG. 3(B) shows a RHEED diffraction pattern, from which it is seenthat the epitaxial thin film is of the single crystal.

[0061] Next, the rare-earth site (Re) of the target composition issubstituted successively with the other rare-earth elements: La, Pr, Nd,Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu to prepare different targetssuccessively. Using these targets and upon the RHEED diffraction patterndetermination, it has been confirmed for all these other rare-earthelements as well that a single crystal thin film is grown epitaxially onthe (010) crystallographic surface of a GdCa₄O(BO₃)₃ substrate which hasbeen superflattened to an atomic level.

[0062]FIG. 4 is a graph showing minimum substrate temperatures requiredfor the epitaxial growth of a single crystal film for differentrare-earth elements. In the graph, the abscissa axis represents the ionradius of a rare-earth element and the ordinate axis represents thesubstrate temperature.

[0063] As is apparent from the Figure, while the minimum substratetemperature required for the epitaxial growth of a single-crystal thinfilm varies with a different rare-earth element, a ReCa₄O(BO₃)₃ familyoxide single crystal thin film can be obtained for every rare-earthelement if the substrate temperature is raised above the minimumtemperature required therefor.

[0064] Mention is next made of SHG property of a Gd_(1-x)Y_(x)Ca₄O(BO₃)₃thin film made by the abovementioned method of making a ReCa₄O(BO₃)₃family oxide single crystal thin film in accordance with the presentinvention.

[0065]FIG. 5 shows results of the measurement of the SHG property of anoxide single crystal thin film having a composition expressed bychemical formula: ReCa₄O(BO₃)₃ where Re is one or more rare-earthelements, in accordance with the present invention.

[0066]FIG. 5(A) shows an optical system for the measurement of the SHGproperty.

[0067] In the Figure, a laser light beam of 1.06 μm from a light source1 made of a Nd: YAG laser passes through an aperture 3 and is incidentvertically on a half-wavelength plate 4. Then, the laser beam 2 whosedirection of polarization is set at a given angle by the rotation by arotary drive 5 of the half-wavelength plate 4 in a plane vertical to theoptical axis is incident vertically on the surface of aGd_(1-x)Y_(x)Ca₄O(BO₃)₃ thin film 6 whereby an SHG light 7 thereof isformed. The SHG light 7 and the laser light 2 as its fundamental waveare guided via a mirror 8 into a prism 9 and an ND filter 10 where thefundamental wave component is cut to allow only the SHG light 7 to beincident on a photo detector 11 where its strength is measured.

[0068]FIG. 5(B) is a graph showing that the intensity of the SHG lightformed by the Gd_(1-x)Y_(x)Ca₄O(BO₃)₃ thin film 6 is measured as afunction of the rotary angle of polarization. In the graph, the abscissaaxis represents the rotary angle of polarization (°) of the laser light2 incident on the Gdl-xYxCa₄O(BO₃)₃ thin film 6 while the ordinate axisrepresents the measured intensity of the SHG light 7 plotted inarbitrary scale () and the normalized intensity of the SHG light 7 (∘).

[0069] As is seen from FIG. 5(B), the SHG light generated by theGd_(1-x)Y_(x)Ca₄O(BO₃)₃ thin film 6 has four (4) peaks generated in arotary angle of polarization 360°, namely in one rotation. These fourgenerated peaks correspond to the directions of polarization in whichthe phase matching between the fundamental wave and the SHG light ismet. A SHG light generated by the fundamental wave incident on the (010)crystallographic surface of a bulk Gdl-xYxCa₄O(BO₃)₃ single crystal isalso found to have four such peaks generated in one rotation.

[0070] From the above, it is seen that a Gd_(1-x)Y_(x)Ca₄O(BO₃)₃ thinfilm epitaxially grown in accordance with the present invention is asingle-crystal thin film having a crystallographic structure like thatof the bulk Gdl-xYxCa₄O(BO₃)₃ single crystal. It has been confirmed,however, that the substrate GdCa₄O(BO₃)₃ crystal does not generate a SHGlight with the abovementioned angle of incidence.

INDUSTRIAL APPLICABILITY

[0071] Making use of a superflattening method for oxide crystal surfacesaccording to the present invention allows a surface of an oxide crystalthat is soluble neither with acid nor with alkaline to be superflattenedto an atomic level.

[0072] Also, making use of a method of making a ReCa₄O(BO₃)₃ familyoxide single crystal thin film according to the present invention allowsa single crystal thin film of any oxide in the ReCa₄O(BO₃)₃ family to bemade.

[0073] Also, a ReCa₄O(BO₃)₃ family oxide single crystal thin filmaccording to the present invention has nonlinear optical property and iscapable of generating a SHG and a THG light efficiently.

[0074] Further, making use of a superflattening method for lightincident/emitting surfaces according to the present invention prevents alight incident/emitting surface from breaking and having any opticaldamage.

[0075] Further, making use of a crystal defect assessing methodaccording to the present invention allows assessing the quality of anoxide crystal that is soluble neither with acid nor with alkaline.

[0076] These aspects of the present invention are applicable to thetechnical fields that require ultraviolet laser light, especially ascore technologies of optical devices applied to optical informationprocessing, optical communication or the like.

What is claimed is:
 1. A method of superflattening a surface of an oxidecrystal that is soluble neither with an acid nor with alkaline,characterized in that it comprises the steps of: reducing the surface ofthe oxide crystal with a reducing; agent; dissolving the reduced oxidecrystal surface with an aqueous acid or alkaline solution; andheat-treating in the atmosphere the oxide crystal with its reducedsurface dissolved, whereby a surface of the oxide crystal solubleneither with acid nor with alkaline is flattened on an atomic level. 2.A method of making a ReCa₄O(BO₃)₃ system oxide single crystal thin film,characterized in that it comprises the steps of: reducing with areducing agent a surface of an oxide single crystal having a compositionexpressed by chemical formula: ReCa₄O(BO₃)₃ where Re represents one ormore rare earth element; dissolving the reduced oxide single crystalsurface with an aqueous acid or alkaline solution; heat-treating in theatmosphere the oxide single crystal with its reduced surface dissolved,thereby superflattening a surface thereof; and epitaxially growing aReCa₄O(BO₃)₃ thin film on the superflattened surface.
 3. An oxide singlecrystal thin film having a composition expressed by chemical formulae:ReCa₄O(BO₃)₃ where R is one or rare earth elements, characterized inthat it is a thin film of said composition epitaxially grown on asurface of an oxide single crystal having a composition expressed bychemical formula: ReCa₄O(BO₃)₃ and superflattened.
 4. An oxide singlecrystal thin film as set forth in claim 3, characterized in that theoxide single crystal thin film having a composition expressed bychemical formula: ReCa₄O(BO₃)₃ is a single crystal thin film that has anmonoclinic biaxial crystalline structure belonging to point group m andspace group Cm.
 5. An oxide single crystal thin film as set forth inclaim 3, characterized in that the oxide single crystal thin film havinga composition expressed by chemical formula: ReCa₄O(BO₃)₃ has anonlinear optical property.
 6. A method of flattening a lightincident/emitting surface, characterized in that the surface is of anoxide optical crystal soluble neither with acid nor with alkaline andthe method comprises the steps of: reducing a light incident/emittingsurface of the oxide optical crystal with a reducing agent; dissolvingthe reduced oxide optical crystal surface with an aqueous acid oralkaline solution; and heat-treating in the atmosphere the oxide opticalcrystal with its reduced surface dissolved, to thereby flatten the lightincoming and outgoing surface, whereby a light-incident/emitting surfaceof the oxide optical crystal is flattened.
 7. A crystal defectassessment method, characterized in that it comprises the steps of:reducing with a reducing agent a surface of an oxide crystal which issoluble neither with acid nor with alkaline; and dissolving defects inthe reduced oxide crystal surface with an etching solution for etchingsaid defects selectively, thereby forming etch pits of the oxide crystalfor assessment of the crystal defects.